IgA nephropathy is a disease that was reported by Berger, et al., in 1968, and is the most frequent kidney disease among chronic glomerulonephritis other than diabetic nephropathy (for example, see Non-Patent Document No. 1). IgA nephropathy is a disease with a poor long-term prognosis and it is estimated that about 25% of patients with terminal renal failure who are in need of dialysis treatment have IgA nephropathy as the original disease. Although there are methods for delaying the progress of IgA nephropathy, no therapeutic method of treating the disease has been found. Recently, however, a treatment method has been established by Hotta, et al., by which complete remission is achieved if the treatment is started at an early stage.
Meanwhile, according to the “Joint Committee of the Specified Progressive Diseases Investigative Research Division for Progressive Renal Diseases of the Ministry of Health and Welfare and the Japanese Society of Nephrology”, while an analysis of urine (continuous microscopic hematuria, continuous or intermittent proteinuria, macroscopic hematuria) and a blood test (blood serum IgA value is 350 mg/dL or more) are employed as supplementary diagnostic criteria, confirmation of IgA precipitation in a glomerular mesangial area based on kidney biopsy is the only means for definite diagnosis of IgA nephropathy. However, as kidney biopsy requires hospitalization for a week or so and is a risky method accompanied by severe bleeding, the diagnosis rate is not necessarily high. Thus, a detection method by which early IgA nephropathy can be conveniently and safely diagnosed is needed.
Further, although methods such as those using an IgA-fibronectin complex are known as diagnosis methods for IgA nephropathy (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2000-241431, Japanese Patent (JP-B) No. 2592121, and Cederholm et al., Proc. Natl. Acad. Sci., 85: 4865-8, 1988), until now no practical method has been developed.